This paper describes the elements, structure, course and outcomes of a negotiation simulation exercise for the University of Eastern Finland – UN Environment Course on Multilateral Environmental Agreements (MEAs), held in Siena on 22-23 October 2019. The scenario for the negotiation simulation focused on marine biodiversity of areas beyond national jurisdiction. These areas comprise of the high seas and the international seabed area, which are jointly referred to as areas beyond national jurisdiction (ABNJ). These areas cover more than 50 per cent of the oceans. Marine biodiversity underpins a variety of ecosystem services of crucial importance to humans, but faces growing threats due to, inter alia, accelerating exploitation of these resources, growing shipping and fishing activities, developed seabed mining and bioprospecting, climate change and ocean acidification.